Ichiro Yonenaga

Crystal growth of dislocation-free LiNbO3 single crystals by micro pulling down method

Abstract LiNbO 3 micro single crystals were succesfully grown by a crystal growth method using a micro nozzle in the bottom of a crucible. By this method, crack-free LiNbO 3 single crystals have been grown along the x, y and z axes having diameters of 60–800 μ m, independent of the growth axis, growth rate and melt composition. Very high growth rates of up to 90 mm/h have been used without lessening of crystal quality. The grown crystals were revealed to be free of dislocations and subgrain boundaries up to 500 μ m in diameter, as determined by X-ray topography and etching technique.

Effects of nitrogen on dislocation behavior and mechanical strength in silicon crystals

The characteristics of dislocation behavior and mechanical strength in tensile tests are investigated on the silicon crystals that are doped with nitrogen at the time of crystal growth by the floating‐zone technique. These are compared with those in the usual floating‐zone‐grown silicon crystals. Nitrogen atoms dispersed in a silicon crystal are shown to have no influence on the velocities of dislocations in motion in the temperature range above 600 °C. Dislocations in the nitrogen‐doped crystal are, however, immobilized while the crystal is kept under a low or zero applied stress at elevated temperatures. Like Czochralski‐grown silicon, nitrogen‐doped silicon shows a much higher yield strength than usual floating‐zone‐grown silicon when crystals are dislocated. It is concluded that interstitial nitrogen atoms bring about the hardening of silicon crystals through locking of dislocations upon congregating on the latter.

Impurity effects on the generation, velocity, and immobilization of dislocations in GaAs

The effects of various kinds of impurities, Al, In, Si, Te, and Zn, on the dynamic behavior of dislocations in GaAs crystals are investigated. The dislocation generation from a surface scratch is suppressed by the doping of the impurities. Generally, the suppression effect of any given kind of impurities depends on the type of dislocations to be generated. However, no systematic dependence on the size misfit or the electrical activity of the impurity atom is found in the suppression efficiency. The immobilization of originally fresh dislocations due to aging at elevated temperature is investigated. The results are interpreted to be due to the gettering of impurities by the dislocations. The feature of the suppression effect in dislocation generation is well correlated to that of the dislocation immobilization due to impurity gettering. It is concluded that the suppression of dislocation generation by impurity doping originates from dislocation immobilization due to impurity gettering. Isovalent impurities...

Mechanical strength of silicon crystals as a function of the oxygen concentration

Mechanical strengths of silicon crystals involving oxygen at various concentrations up to 1018 atoms/cm3 are investigated with the use of crystals grown by the Czochralski technique in magnetic field. Oxygen atoms dispersed on interstitial sites at any concentration have almost no influence on the dislocation processes that control the mechanical behavior of a dislocation‐free crystal. Oxygen atoms in a dislocated crystal lock dislocations effectively and result in the strengthening of the crystal. Locking of dislocations becomes more effective as the concentration of oxygen in the crystal increases. Softening of crystals due to precipitation of oxygen at elevated temperature occurs rapidly in highly concentrated crystals with oxygen, but is almost absent or takes place slowly in crystals of oxygen concentrations lower than about 5×1017 atoms/cm3.

Characteristics of dislocations in ZnO layers grown by plasma-assisted molecular beam epitaxy under different Zn∕O flux ratios

We have investigated the characteristic of the dislocations in the ZnO layers grown on c sapphire by the plasma-assisted molecular beam epitaxy under the different Zn∕O flux ratios. The ZnO layers were characterized by the transmission electron microscopy (TEM) and the high-resolution x-ray diffraction (HRXRD). The TEM and HRXRD experiments revealed that the major threading dislocations (TDs) in the ZnO layers are the edge dislocations running along the c axis with Burgers vector of 1∕3⟨11–20⟩. The TD densities are determined to be 6.9×109, 2.8×109, and 2.7×109cm−2, for O-rich, stoichiometric, and Zn-rich grown ZnO, respectively. Different from the O-rich grown ZnO where the dislocations run along the c-axis, several dislocations in the stoichiometric and the Zn-rich grown ZnO are inclined to 20°∼30° from the c-axis. By considering the slip system in the wurtzite-structure ZnO, the glide planes of the dislocations are close to (10-10) for the O-rich grown ZnO and close to (10-11) for the stoichiometric an...

Influence of oxygen precipitation along dislocations on the strength of silicon crystals

The mechanical strength of dislocated crystals of Czochralski‐grown silicon as influenced by the precipitation of oxygen impurities on dislocations was investigated. The yield strength increases during the early stage of precipitation of silicon oxide, but decreases remarkably during the later stage of precipitation. The enhancement of the yield strength is brought about by the immobilization of dislocations due to locking by closely aligned precipitates along the dislocations during the early stage of precipitation. The locking effect diminishes during the late stage when precipitates on dislocations coalesce with the generation of free portions of dislocations with large separation.

Czochralski growth of Ge1 − xSix alloy crystals

Single crystal Ge1 − xSix alloys of composition 0 < x < 0.64 were grown by the Czochralski method. In particular, a single crystal 20 mm in diameter and 60 mm in length with variable composition 0.004 < x < 0.03 along the growth direction, and a crystal 12 mm in diameter and 20 mm in length with composition 0.21 < x < 0.25 were obtained. The crystals became Ge rich along the growth direction as determined by energy dispersive X-ray (EDX) spectroscopy. Fine striation structures in the crystals were observed by X-ray topography.

Effects of In impurity on the dynamic behavior of dislocations in GaAs

The effects of In impurity on the dynamic behavior of dislocations in GaAs crystals are investigated in comparison with those of Si impurity. Indium, even at a concentration of the order of 1020 cm−3, is found to affect the velocity of moving dislocations only slightly while Si impurity at a concentration of the order of 1018 cm−3 reduces it by one or two orders of magnitude. However, α dislocations are easily immobilized by In atoms while they are at rest by gettering the latter. Immobilization of β dislocations due to In gettering is much weaker than that of α dislocations. An interpretation is given of how such a difference in locking by In impurity comes about between α and β dislocations. The function of Si impurity to lock both α and β dislocations is stronger than that of In impurity on α dislocations. Nevertheless, the locking effect due to Si impurity manifests itself only in the high‐temperature range. This seems to be related to the low diffusivity of Si impurity in GaAs crystals.The effects of In impurity on the dynamic behavior of dislocations in GaAs crystals are investigated in comparison with those of Si impurity. Indium, even at a concentration of the order of 1020 cm−3, is found to affect the velocity of moving dislocations only slightly while Si impurity at a concentration of the order of 1018 cm−3 reduces it by one or two orders of magnitude. However, α dislocations are easily immobilized by In atoms while they are at rest by gettering the latter. Immobilization of β dislocations due to In gettering is much weaker than that of α dislocations. An interpretation is given of how such a difference in locking by In impurity comes about between α and β dislocations. The function of Si impurity to lock both α and β dislocations is stronger than that of In impurity on α dislocations. Nevertheless, the locking effect due to Si impurity manifests itself only in the high‐temperature range. This seems to be related to the low diffusivity of Si impurity in GaAs crystals.

The Origin of the Difference in the Mechanical Strengths of Czochralski-Grown Silicon and Float-Zone-Grown Silicon

The mobility of individual dislocations was measured by means of in-situ observations, including the use of X-ray topography, for both Czochralski- and float-zone silicon crystals. No difference in mobilities was found between the two types of crystals. Stress-strain characteristics were also measured for both types of crystals. On the basis of observed facts, it is concluded That the difference in the mechanical strengths of the two types of silicon crystals is associated with the locking effect of dislocations by oxygen atoms.

Mechanical properties of GaAs crystals

Mechanical properties of GaAs crystals grown by the liquid encapsulated Czochralski technique and the boat technique are investigated by means of compression tests. Stressstrain characteristics of a GaAs crystal in the temperature range 400°–500°C are very similar to those of a Si crystal in the temperature range 800°–900°C. This seems to reflect the fact that the dislocation mobility in a GaAs crystal in the former temperature range is comparable to that in a Si crystal in the latter temperature range. Dislocations in GaAs crystals are found to be easily immobilized at an intermediate temperature due to gettering of impurities and/or impurity-point defect complexes. In comparison to a Si crystal, the surface of a GaAs crystal seems to involve irregularities that act easily as effective generation centers for dislocations. Thus the magnitude of the yield stress of an aged GaAs crystal is controlled by the surface condition and is not influenced by the density of dislocations involved in the crystal. The socalled steady state of deformation is realized in a GaAs crystal in the deformation stage after the lower yield point as in Si and Ge crystals. Dislocation distributions in a deformed GaAs crystal observed by transmission electron microscopy is very similar to those in deformed Si and Ge crystals.